1. Field of the Invention
The present invention relates to an optical element capable of controlling a light transmittance, light absorption and a light scattering property thereof reversibly in response to an electrical stimulus and to a method of producing the optical element. More particularly, the invention relates to an optical element that can be used in a light controlling element, a display element, a recording element, a sensor or the like each of which utilizes an electrical stimulus, and relates also to a method of producing such an optical element.
2. Description of the Related Art
An optical element technology has been known in which light is controlled or color is developed by controlling a light transmitting amount or a light scattering property by utilizing a stimulus-responsive polymer. Such a stimulus-responsive polymer reversibly swells or contracts (changes its volume) by a change in pH value, a change in ion intensity, absorption/desorption of a chemical material or a change in composition of a solvent therein, or application of a thermal, optical or electrical stimulus thereto.
For example, an element has been proposed in which an optical scattering property is controlled by changing a difference in refractive index between a solvent and a polymer due to swelling and contraction of a polymer that absorbs or releases a liquid according to a change in temperature to thereby present a display (for example, see Japanese Patent Application Laid-Open (JP-A) Nos. 61-151621, 62-925 and the like). Furthermore, the following display elements have been proposed: an element presenting a display according to a change in light scattering property of a polymer that absorbs or releases a liquid by an electrical stimulus (see JP-A No. 4-134325); an element presenting a display according to a change in light scattering property of a polymer caused by a change in pH value due to doping or dedoping of an ion to or from a conductive polymer in the polymer (see Japanese Patent Application Publication (JP-B) No. 7-95172); and an element presenting a display in cloudiness and transparency by screening, reflecting, or scattering light or by controlling a light transmittance with swelling or contraction of a polymer that absorbs or releases a liquid by an action of an electric field (see JP-A No. 5-188354).
The inventors have proposed a light controlling material in which a stimulus-responsive polymer comprises a pigment at a saturated absorption concentration or higher or a light scattering material at a saturated scattering concentration or higher (for example, see JP-A No. 11-236559). The light controlling material has strong points that the light controlling material provides a reversible large change in color and is easily applied to multicolor display. An optical element having a light controlling layer consisting of particles (polymer particles) of the light controlling material fixed on a substrate is also proposed (see, for example, JP-A No. 2001-350163). By fixing polymer particles on a substrate as recited above, aggregation of the polymer particles caused by repetition of swelling and contraction can be prevented. In addition, it is possible to manufacture an element that has an excellent color developing characteristic or light scattering characteristic when the polymer swells and which has a good contrast ratio.
The present inventors have further found a phenomenon that a chargeable polymer having a specific composition repeatedly swell or contract in an insulating organic swelling solution in accordance with a change in an electric field. Thus the inventors have manufactured an element utilizing this phenomenon (see, for example, JP-A No. 2003-147221). Such an electric-field-responsive chargeable polymer solves problems of a conventionally known current-responsive polymer such as a high power consumption, a low durability, and generation of bubbles by electrolysis of a swelling liquid. Accordingly, the electric-field-responsive chargeable polymer is expected to be applied to optical elements such as a display element, a light controlling element, and a light switch element and to a micro-machine. It has been known that the electric-field-responsive behavior of this specific chargeable polymer takes place when the chargeable polymer particles are fixed on a substrate such as an electrode. That is, it is important that chargeable polymer particles should be fixed on a substrate.
However, by a further study of the optical element having such an electric-field-responsive chargeable polymer on a substrate by the inventors, it was found that the polymer also has a problem. That is, the polymer changes its shape, contacts with the surface of a substrate, and adheres to the substrate while swelling and contraction (a volume change) of the polymer repeats in response to changes in an electric field, thereby decreasing a volume change amount as time passes.